The present invention relates to a wavelength filter.
As optical devices that have advantages in miniaturization and mass production have been developed, Si waveguides using silicon as material of the waveguides have gained attention in recent years.
With regard to a Si waveguide, an optical waveguide core which substantially serves as a light transmission path is formed using silicon as material thereof. Then, a circumference of the optical waveguide core is covered by cladding whose material is, for example, silica having a lower refractive index than silicon. Since a difference between refractive indices of the optical waveguide core and the cladding is very significant in the above-described configuration, it is possible to tightly confine light to the optical waveguide core. As a result, a small-size curved waveguide whose bend radius is reduced to, for example, about 1 μm can be realized. Optical circuits with a similar size to electronic circuits can be produced with such small-size curved waveguides, which brings an advantage of overall miniaturization of an optical device.
In addition, a manufacturing procedure of a semiconductor device such as a complementary metal oxide semiconductor (CMOS) can be applied to manufacturing of Si waveguides. Therefore, opto-electronic fusion (silicon photonics) in which an electronic function circuit and an optical function circuit are consolidated on a chip is expected to be realized.
In a passive optical network (PON) which uses wavelength division multiplexing (WDM), different reception wavelengths are allocated to respective optical network units (ONU). An optical line terminal (OLT) generates downlink optical signals with respect to of the respective ONUs having transmission wavelengths corresponding to reception wavelengths of transmission destinations, and multiplexes and transmits the signals. Each of the ONUs selectively receives an optical signal having a reception wavelength allocated thereto from downlink optical signals multiplexed with a plurality of wavelengths. Each of the ONUs uses a wavelength filter in order to selectively receive the downlink optical signals having a reception wavelength thereof. In addition, a technology of constructing a wavelength filter with the above-described Si waveguide has been realized.
Wavelength filters which use Si waveguides include, for example, a wavelength filter that uses a Mach-Zehnder interferometer and a wavelength filter that uses an array waveguide grating. In addition, ring resonators (disclosed in, for example, Patent Literature 1: JP 2003-215515A, Patent Literature 2: JP 2013-093627A, Patent Literature 3: JP 2006-278770A, and the like) and grating type wavelength filters (disclosed in, for example, Patent Literature 4: JP 2006-330104A, and the like) or directional coupler type wavelength filters (disclosed in, for example, Patent Literature 5: JP 2002-353556A, and the like) can be exemplified as wavelength filters which use Si waveguides. These wavelength filters have an advantage of causing output wavelengths to be variable by including electrodes and using heat from the electrodes.
Here, the grating type or directional coupler type wavelength filters output light having a unimodal wavelength peak. Conversely, the ring resonators output light having multimodal wavelength peaks. For this reason, the ring resonators have an advantage of expanding wavelength variable regions by exploiting the Vernier effect in which a plurality of wavelength peaks of output light is used.